The invention relates to a method for the determination of those points on a specimen which carry a specific signal frequency. A scanning microscope is employed for the purpose of scanning the specimen. The scan rate is defined by a scan generator and by a deflection device controlled by the scan generator. A secondary electrical signal is derived from the respective scanned point via a detector. This electrical signal is supplied to an evaluation circuit which selects and outputs a signal based on and representative of the specific signal frequency.
A method of this type has been proposed in German Patent Application No. P 34 20 272.2, incorporated herein by reference. A particle microscope is employed wherein a particle beam, for example an electron beam, successively scans a group of points on a specimen as a primary beam. The particle beam is pulsed with a variable pulse frequency which deviates from the specific signal frequency by an amount equal to a fixed frequency. A first evaluation circuit is formed of a band-pass filter having a narrow pass-band whose center frequency corresponds to the fixed frequency and of a following demodulator.
A further method of this type is known from Proc. of Journee d'Electronique, 1983 "Testing Complex Integrated Circuits: A Challenge", published by the Swiss Federal Institute of Technology, Lausanne, Switzerland, pages 283 through 298, incorporated herein by reference. The detection of a specific signal frequency at the points of an integrated circuit is executed herein by use of a scanning electron microscope and of a "lock-in amplifier" serving as a first evaluation circuit. A signal having the desired signal frequency is filtered out from a potential contrast signal acquired at a point. It is filtered out with the assistance of the "lock-in" amplifier. The intensity of this signal is then displayed on the picture tube of the microscope as an intensity fluctuation.
Given the methods cited above, the maximum speed with which a group of points can be successively scanned on the specimen is limited by the band width of the first evaluation circuit. This band width must be as great as possible for a fast measurement. This likewise is desirable in view of the lowest possible load on the specimen due to the electron beam. However, the sensitivity of the method decreases with increasing band width, since the signal-to-noise ratio of the output signal of the first evaluation circuit deteriorates. In order to achieve a short measuring time, therefore, a reduction of the sensitivity or of the spatial resolution of the scanned points on the specimen must be accepted.